The present invention relates to a system for heating or converting at least one medium, having an evaporator, a reactor and a heat exchanger.
Gas generating systems for fuel cells contain various components operated at different temperature levels, through which the flow takes place in a specified sequence. The various components have partially separate apportioning units, and are either linked in separate housings or by pipe systems, or are coupled in a housing to form multifunction units.
U.S. Pat. No. 5,015,444 discloses a plate reactor for reforming hydrocarbons which has upper and lower end plates, as well as plates in between, which are arranged in a stack and define media spaces. The media flows over the plates (which consist of massive metal plates), parallel to the plane of the plates. The catalyst is placed in the form of a charge or in the form of a coating.
Japanese Patent Document JP-7-126001 discloses a system in which the essential components of a gas generating system are joined to form a compact unit.
European Patent Document EP 0 906 890 A1 discloses a system of the above mentioned type for generating hydrogen from hydrocarbons or alcohol while feeding a reaction mixture comprising hydrocarbon or alcohol and water to a catalyst. The catalyst is produced by pressing at least one catalyst powder into a very compressed layer which forms a molded body. Thus, the reaction mixture can be pressed through the catalyst layer, with a corresponding pressure drop.
Other components of a gas generating system which are constructed of pressed layers through which the medium flows are known from German Patent Documents DE 197 20 294 C1 (evaporator/reformer), DE 199 01 301.2 (reformer/gas purification, no prior publication), DE 198 47 987.5 (Process for Producing Such Layers, no prior publication), DE 198 32 625.4 (Production of a Stacked Body, no prior publication).
Finally, in German Patent Document DE 195 34 433 C1, the medium which is to be reformed flows over a pressed catalyst layer for a methanol reforming reactor. The catalyst layer is formed by a compressed metal-foam carrier layer in whose pores the catalyst material is fixed. After charging with the catalyst material, the layers are compressed to fix the catalyst material.
Particularly for mobile applications, a high degree of integration must be achieved in order to keep the mass of such systems as small as possible, while minimizing the costs. When such a system is used, the reactions taking place therein are coupled to narrow operating temperature ranges. Particularly when used in mobile applications, such as vehicles which are driven by fuel cell systems, temperature fluctuations and cold starting conditions are critical in the case of such modular plate structures.
It is therefore an object of the invention to provide a system for heating and/or converting at least one medium, in which different function areas can be combined while creating a degree of integration which is as high as possible.
This and other objects and advantages are achieved according to the invention by subdividing the stack formed of pressed layers into a number of function areas. In this manner, a high degree of integration can be achieved, for example, in a system for generating hydrogen from methanol and water. It is possible, for example, to implement an evaporator and a catalytic reactor, which, in turn, may be divided into different stages, within one plate stack.
A low overall mass of the hot reaction area reduces the amount of heat required for the cold start. In addition, a compact shape of the hot reaction area facilitates thermal insulation.
A preferred embodiment of the invention has insulating plates which divide the layers into thermally essentially mutually insulated function areas. By arranging insulating layers between the individual layers arranged in a stack, individual thermally mutually insulated function areas can be defined simple manner in which, for example, different temperatures exist. This measure can be implemented at very reasonable cost.
The layers can also be arranged expediently between a lower and an upper end plate, favorably influencing the mechanical stability of the system according to the invention.
According to another preferred embodiment of the system according to the invention, the insulating plates extend essentially in parallel to the reactive layers of the system. As a result, areas of different functionality can easily be defined in a flow direction of a medium to be reacted.
Insulators can also be provided between the end plates and the adjacent layers, thereby ensuring that the end plates can be thermally uncoupled from the plates forming the media spaces, so that a desired operating temperature can be reached more rapidly in the media spaces. In systems which include components that have a high thermal capacity (such as massive end plates made of steel), and which, in addition, require a mechanical support when installed into a system to be operated, the influence of the unavoidable thermal coupling to the mechanical supports is reduced in this manner.
Advantageously, the end plates are made of aluminum. While their weight low, such end plates have a good mechanical stability as long as they have a significantly lower temperature than those in the hydrogen-releasing zone.
According to another preferred embodiment of the invention, the system has devices for clamping the layers between the two end plates, achieving a simple and stable construction, In this manner, a good sealing effect can also be achieved if seals are provided, for example, between the individual layers. The devices for clamping the layers between the end plates can apply pressure force to the seals. Such clamping devices are expediently constructed as tie rods, tension members or similar devices. Tie rods are available at reasonable cost and have been found to be robust and reliable in practice.
Advantageously, edge areas of the layers are sealed off from the environment, preventing the emission of reaction products into the ambient air, which would reduce the efficiency achievable by means of the system according to the invention. The layer edges can be sealed, for example, by soldering or metal spraying, or by using other materials.
It was further found to be expedient to provide higher temperature function areas in the interior of the stack, so that they can be optimally thermally insulated.
It was further found to be expedient to provide insulation which surrounds the entire stack, so that low heat losses (and therefore high efficiencies) can be achieved.
It is also advantageous to arrange the end plates and the clamping devices outside the thermally insulated layer.
According to a particularly preferred embodiment of the system according to the invention, educt ducts extend through at least a portion of the layers, so that individual function areas can be selectively acted upon by respective educts. Also, connecting ducts can be provided which extend through at least a portion of the layers and which guide educts or products from at least a first function area into at least a second one; In addition, product ducts can be provided for removing educts or reaction products heated or formed in the individual function areas from the respective function areas. As a result of such a selective admission, function areas can be defined in a simple manner, and a particularly high degree of integration can be achieved. The combination of this measure with the construction of insulating plates was found to be particularly advantageous for providing thermally mutually insulated function areas. For example, several identical function units can be provided within the stack which are thermally mutually insulated. As a result of such a modular construction, it is possible (for example, during a cold start) to only act upon (for example, heat or keep warm) individual function areas, while nevertheless ensuring an operable unit.
Expediently, for acting upon different function areas by means of the same educt, different educt ducts are provided, which communicate selectively with the respective function areas, and/or different product ducts are provided for removing the products from the respective function areas. Such ducts can essentially extend parallel to one another and, as a result of the corresponding construction of openings, can communicate only with certain function areas.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.